There is a lot of debate within the aquarium hobby about whether or not air stones and air bubbles actively aerate or simply increase flow and surface agitation.
Air bubbles definitely do actively aerate the water. Gas exchange occurs at ANY air-water surface. The oxygen doesn’t stop and think, ‘Hey, this is just a bubble, I can’t cross here, I have to wait until I am in the air above the water.’ This is well known and accepted in other industries, but for some reason it just can’t seem to catch on in the aquarium hobby (maybe because none of us are professionals, just hobbyists, regurgitating what we hear other hobbyists claim?). Bubble aeration is a strong tool in both aquaculture (real aquaculture, not hobbyist aquariums), hydroponics, and sewage treatment because the bubbles themselves actively aerate the water column. If this wasn’t true they could just use pumps moving water aimed at the surface to get surface agitation.
I have seen multiple tanks saved multiple times simply because they had an air stone. This includes both of the 220 gallon tanks at the shop I was running. One housed a two foot pacu, two foot achara catfish, and eighteen inch arowana. The other housed a lot of different African cichlids (mbunas, peacocks, and open water haps) as well as bristlenose plecos, Synodontis spp., and bichirs (polypterus spp.). Both of these tanks have gone a week without any filtration multiple times (cichlids spitting sand into the filter intake, filter not turning back on after being cleaned, filter not being plugged in after being cleaned, etc.) yet experienced no losses because the air stones alone provided enough aeration. I have also seen tanks that started losing fish within only a couple hours of a filter stopping because they had no air stone.
The only hope someone has of proving the idea that bubbles don’t actively aerate is showing that there is a minimum reaction time required between the air and water for aeration to take place. If this were true then that would also apply to water flow at the surface of the tank. If there is a minimum reaction time then strong flow at a tank’s surface would inhibit aeration by not allowing a long enough reaction time. Since this is obviously not the case it should be apparent that bubbles themselves do actively aerate. Smaller bubbles create a lot more surface area per volume of air. Larger bubbles have less surface area per volume of air but create more flow than smaller bubbles. Both can greatly increase overall aeration in an aquarium.
The basic assumptions/issues in question are:
1-Bubbles add little surface area
2-They rise too quickly for aeration to take place
1-Every single bubble is a sphere of surface area. With a strong air pump and fine bubbles you can actually match and exceed the top surface area of the tank. Any surface area counts. The surface area in bubbles does not collect a film that inhibits aeration the way the surface of the tank does, so it has the most potential for aeration. Because the bubbles are moving they are constantly exposed to water that has less oxygen than the water they had just been exposed to, meaning the potential for aeration is very high (like the cross current flow of blood in fish gills).
2-Bubbles do not rise too quickly for aeration to take place. This would require there to be a minimum contact time between air and water for aeration to take place, which is not the case. Aeration is diffusion, gasses passing from air to water and from the water to the air, and this takes place almost instantly. More contact time increases it, but there is enough contact time for aeration as bubbles rise to the surface. If this were not true, if there was a minimum contact time, then it would be possible to have TOO MUCH flow at the surface of a tank because the water would simply pass by the surface too fast for aeration to take place. This is obviously not the case.
A weak air pump on a relatively large tank may not provide a significant amount of aeration. But this is like saying that filters don’t filter because if the filter is too small it doesn’t filter enough.
The use of CO2 gas in planted tanks shows that aeration does occur in bubbles. Even when only utilizing one bubble at a time you can very effectively increase CO2 concentration in an aquarium. Granted it is pure CO2 gas so the concentration difference is high which will increase the rate and almost any time CO2 gas is used some device to increase the time the bubble spends in the water is used as well, this still clearly shows that aeration actively occurs at bubbles. To think that CO2 gas one bubble at a time can create gas exchange but a torrent of air bubbles can’t actively aerate is blindly biased.
The National Zoo in D.C. has a massive air stone assembly in their stingray/arowana pool. This is obviously for the aeration it provides since it is not for looks in their otherwise very natural setups. This is something you will see over and over in scientific settings where things are used purely for function.
I have also seen many tanks have more flow created by the air stones than by the filters, both HOBs and canisters. A good strong air pump can create a lot of very natural flow. A HOB, canister, powerhead, etc. create a lot of flow in a small area or stream but can fail to get the entire water column moving. Air stones can get the entire water column moving. The best example of this was a 220 gallon tank at the shop I was running. We added an air stone and it provided more flow than the Fluval FX5. The FX5 moved water along the top of the front of the tank. When we added the air stone it created so much flow that is stirred up debris that had been sitting on the bottom all over the tank. This debris was visible in the water column so we could see the flow it was creating. The flow was impressive to say the least. The entire water column was moving. From end to end, front to back, top to bottom of this 6’x2’x30″ tank.
It is nothing short of blindly ignorant to think that bubbles don’t actively aerate. Other industries and scientific functions use bubbles to aerate. We in the hobby are the only ones blindly denying the value of these cheap, effective, and sometimes invaluable pieces of equipment.